Methods of forming and operating back-side trap non-volatile memory cells

ABSTRACT

Methods of forming and operating a back-side trap non-volatile memory cell. Method of forming a back-side trap non-volatile memory cell include forming a trapping material, forming two or more sub-layers of dielectric material on the trapping material, wherein a conduction band offset of each sub-layer of dielectric material is less than the conduction band offset of the material upon which it is formed, and forming a channel region on the two or more sub-layers of dielectric material. Methods of operating a back-side trap non-volatile memory cell include programming the memory cell via direct tunneling of carriers through an asymmetric band-gap tunnel insulator layer having two or more sub-layers formed beneath a channel region and having layers of material of increasing conduction band offset, and trapping the carriers in a trapping layer formed under the tunnel insulator layer.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/177,383, titled “BACK-SIDE TRAPPED NON-VOLATILE MEMORY DEVICE,” filedJul. 22, 2008 now U.S. Pat. No. 7,851,827 (Allowed), which is acontinuation of U.S. patent application Ser. No. 11/157,361, titled“BACK-SIDE TRAPPED NON-VOLATILE MEMORY DEVICE,” filed Jun. 21, 2005, nowU.S. Pat. No. 7,402,850, each of which is commonly assigned andincorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to integrated circuits and inparticular the present invention relates to non-volatile memory devices.

BACKGROUND

Memory devices are typically provided as internal storage areas in thecomputer. The term memory identifies data storage that comes in the formof integrated circuit chips. There are several different types of memoryused in modern electronics, one common type is RAM (random-accessmemory). RAM is characteristically found in use as main memory in acomputer environment. RAM functions as a read and write memory; that is,you can both write data into RAM and read data from RAM. This is incontrast to read-only memory (ROM), which permits you only to read data.Most RAM is volatile, which means that it requires a steady flow ofelectricity to maintain its contents. As soon as the power is turnedoff, whatever data was in RAM is lost.

Computers almost always contain a small amount of ROM that holdsinstructions for starting up the computer. Unlike RAM, ROM cannot bewritten to. An EEPROM (electrically erasable programmable read-onlymemory) is a special type non-volatile ROM that can be erased byexposing it to an electrical charge. EEPROM comprise a memory arraywhich includes a large number of memory cells having electricallyisolated gates. Data is stored in the memory cells in the form of chargeon the floating gates or floating nodes associated with the gates. Eachof the cells within an EEPROM memory array can be electricallyprogrammed in a random basis by charging the floating node. The chargecan also be randomly removed from the floating node by an eraseoperation. Charge is transported to or removed from the individualfloating nodes by specialized programming and erase operations,respectively.

Yet another type of non-volatile memory is a Flash memory. A Flashmemory is a type of EEPROM that is typically erased and reprogrammed inblocks instead of a single bit or one byte (8 or 9 bits) at a time. Atypical Flash memory comprises a memory array, which includes a largenumber of memory cells. Each of the memory cells includes a floatinggate field-effect transistor (FET) capable of holding a charge. The datain a cell is determined by the presence or absence of the charge in thefloating gate/charge trapping layer. The cells are usually grouped intosections called “erase blocks.” Each of the cells within an erase blockcan be electrically programmed in a random basis by charging thefloating gate. The charge can be removed from the floating gate by ablock erase operation, wherein all floating gate memory cells in theerase block are erased in a single operation.

The memory cells of both an EEPROM memory array and a Flash memory arrayare typically arranged into either a “NOR” architecture (each celldirectly coupled to a bit line) or a “NAND” architecture (cells coupledinto “strings” of cells, such that each cell is coupled indirectly to abit line and requires activating the other cells of the string foraccess).

Floating gate memory cells are typically programmed by injectingelectrons to the floating gate by channel hot carrier injection (CHE),typically placing the cell in a high threshold voltage state. Floatinggate memory cells can also be erased by hot hole injection from thesubstrate. Alternatively, floating gate memory cells can be programmedand erased by electron tunneling to and from the substrate byFowler-Nordheim tunneling to put the cell in a programmed or erasedthreshold state. Both mechanisms require the generation of high positiveand negative voltages in the memory device and can place high fieldsacross the gate insulation layers with resulting adverse effects indevice characteristics and reliability.

A problem with CHE, hot hole injection and Fowler-Nordheim tunneling isthat the high energy required for their operation damages the oxideinterfacing silicon substrate, reducing memory cell retention,endurance, and degrading transconductance of the FET. In particular, thetunnel insulation layer, which is placed between the channel and thefloating node/charge trapping layer is typically damaged by theprogramming process by having hot carriers injected or tunneled throughit. As high control fields are also typically asserted on the channelduring writing and erasing through the tunnel insulation layer by theapplication of voltage to the control gate of the memory cell, damage tothe tunnel insulator by the hot carrier injection or high fluencetunneling process can significantly affect device characteristics. Hotcarrier injection can generate interface states, degrade devicetransconductance, and enhance device leakage via enhanced short channeleffect, besides affecting charge retention and read-disturb. Hot holeinjection can generate fixed charge trapping centers in the tunnelinginsulators and associated defects in the trapping layer, thus breakingstable bonds and eventually degrading the insulator/dielectricproperties of the device. For a conventional Flash or SONOS non-volatilememory device, the same control gate is also used during a readoperation as a FET to read the state of the memory cell. When the tunnelinsulator (also known as the tunnel oxide) is degraded, the readcharacteristics of the memory cell are also degraded due to thetransconductance degradation and enhanced leakage. This affects the readspeed of the memory cell.

An additional problem in Flash and SONOS memory cell arrays is thatvoltage scalability affects the minimum cell size, and consequently theoverall memory density of any resulting array. As integrated circuitprocessing techniques improve, manufacturers try to reduce the featuresizes of the devices produced and thus increase the density of the ICcircuits and memory arrays. In modern IC circuits and memory arrays, asSONOS transistors and floating gate memory cells are scaled to smallerfeature sizes, the device characteristics of the component transistorsand floating gate memory cells can alter and leave the resulting IC ormemory device non-functional. These issues include, but are not limitedto, short channel effect, signal cross-talk, device programming andoperating voltages, reduced logic windows, oxide punch-through, andcharge leakage and retention.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art formethods and apparatus for a non-volatile memory cell that allows forfeature and voltage scaling, prevents read degradation while providingenhanced retention, speed, endurance, and exhibits increased deviceintegrity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E detail bulk and SOI back-side trap memory cells and banddiagram in accordance with embodiments of the present invention.

FIG. 2 details a system with a memory device in accordance withembodiments of the present invention.

FIGS. 3A and 3B detail NOR and NAND architecture memory arrays inaccordance with embodiments of the present invention.

DETAILED DESCRIPTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration specific preferredembodiments in which the inventions may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that logical, mechanical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The terms wafer and substrate used previously and inthe following description include any base semiconductor structure. Bothare to be understood as including bulk substrate, silicon-on-sapphire(SOS) technology, silicon-on-insulator (SOI) technology,silicon-on-nothing (SON) technology, thin film transistor (TFT)technology, doped and undoped semiconductors, epitaxial layers ofsilicon supported by a base semiconductor, as well as othersemiconductor structures well known to one skilled in the art.Furthermore, when reference is made to a wafer or substrate in thefollowing description, previous process steps may have been utilized toform regions/junctions in the base semiconductor structure. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the claims and equivalents thereof.

Non-volatile memory devices and arrays, in accordance with embodimentsof the present invention, facilitate the utilization of back-sidetrapped floating node memory cells with band-gap engineered gate andgate-insulator stacks with asymmetric tunnel barriers. Embodiments ofthe present invention allow a separate front gate-insulator stack forreading and a separated back gate for programming (write and erase) anda back-side gate-insulator stack for charge storage. Additionally, theembodiments of the present invention allow implementation of thenon-volatile memory device on bulk silicon and SOI or SON substrateswith thick and thin channel regions/floating bodies formed between thefront and back-side gate-insulator stacks (also known as the top orbottom gate-insulator stacks). Embodiments of the present inventionfurther allow for direct tunnel programming and efficient erase withelectrons and holes in back-side trap memory cells, while maintaininghigh charge blocking barriers and deep carrier trapping sites for goodcharge retention and reduces the possibility of damage to thechannel/insulator interface. The direct tunneling program and efficienterase capability reduces damage to the back gate-insulator stack and thecrystal lattice from high energy carriers, reducing write fatigue andleakage issues and enhancing device lifespan. Utilizing the separatefront side gate for reading the back-side trap memory cells also enablefurther scaling of device features with improved lithography throughincreased logic window, decreased susceptibility to short channeleffect, and increased operational speed. Memory device embodiments ofthe present invention are presented that are arranged in NOR or NANDmemory architecture arrays. Memory cell embodiments of the presentinvention also allow multiple levels of bit storage in a single memorycell, and allow for programming and erase with reduced voltages,enhancing endurance and reducing power.

In back-side trap non-volatile memory cell FETs, source and drainregions are formed and are separated by a thin or thick body channelregion with a first gate stack (also known as the front gate stack, topgate stack, or access gate stack) formed over the channel region and/orportions of the source and drain regions. The top gate stack comprises alayer of trap-free insulation and a control gate (the access gate)forming a FET device which allows for the formation of minority carriersin the channel and control of the carrier flow from the source to thedrain through the channel via the voltage applied to the access gate.This top gate FET is typically used exclusively for sensing (reading)the state of the memory cell. A second gate-insulator stack (known asthe back-side gate stack, bottom gate stack, or trapping layer gatestack) is formed beneath the channel region and is employed for chargestorage and programming (writing and erasing). This device elementtypically comprises successive layers of tunnel insulation, a floatingnode trapping layer (or, alternatively, a conductive floating gate), acharge blocking insulation layer and a back-side control gate. Thissecond gate stack, with its electrically isolated trapping layer, allowscharge to be trapped beneath the channel region and affect the thresholdvoltage level of the sensing part (top gate FET) of the memory cell. Theback-side control gate can be formed of aluminum, tungsten, polysiliconor other conductor material and be independently coupled to a controlline or, as is in the case of an SOI or bulk implementation, be formedfrom the substrate. Such back-side trap non-volatile memory cell FETsare also referred to as a vertically integrated “dual gate transistornon-volatile memory devices.”

Conventional SONOS memory cell devices typically employ successivelayers of oxide/nitride/oxide (ONO) as the insulator stack between thesilicon substrate and the heavily doped polysilicon control gate (ormetal gate). The oxide adjacent to the silicon substrate is usuallythinner and acts as a tunnel insulation layer (also known as a tunneloxide) while the thicker oxide adjacent to the control gate acts as thecharge blocking oxide. Trapped charges stored in the device aretypically held at the tunnel oxide-nitride interface and at the bulktrapping centers in the nitride layer. The equivalent oxide thickness(EOT) of the SONOS memory cell gate-insulator stack typically determinesthe program and erase voltage levels of the device. The program/erasespeed, and especially, the erase speed and charge retention are stronglyinfluenced by the tunnel oxide thickness. The logic window between thelogic “1” and the logic “0” memory states (Vt“1”-Vt“0”) is generallydependent on the trapped charge density at the interface and in the bulkof the nitride and consequently increases with increasing thickness ofthe nitride. Due to the high field requirements of the charge transportthrough the tunnel oxide and the relatively low density of deep energytraps in the nitride, the a) voltage scalability, b) program/erasespeed, and c) magnitude of the logic window are generally limited withthe SONOS device ONO insulator stack when an industry standard minimumof ten years of charge retention is required. This is especially true ifprogramming voltage levels are to be scaled.

Regardless of the mechanisms of charge transport employed (CHE, HotHole, or Fowler-Nordheim Tunneling), Floating gate memory cells are evenmore limited in voltage scalability and speed compared to SONOS. This isdue to the fact that the EOT of such device gate-insulator stacks arerequired to be nearly twice that of a SONOS device and the tunnel oxidethickness is two to four times as thick when compared to those of theSONOS device to be able to ensure ten years of charge retention.Floating gate Flash devices typically employ oxide as the tunnel layermedia and a thicker ONO stack on top of the polysilicon floating gate asthe charge blocking layer.

For a single transistor memory cell of either Floating gate type orSONOS type, the control gate acts as both the “read” or “addressing”gate and the gate that controls the operation of programming and erase.During “read” operation, conductance of the memory cell FET determinesthe read speed of the device, which in turn typically depends on thetransconductance and channel width/length (W/L) ratio of the device.During the standby state, device leakage depends on short channeleffects and the stability of the memory states. High voltagerequirements and hot carrier induced degradation of tunnel oxideadversely affect read speed and read disturb, as well as device leakage,as mentioned above.

As the channel length in conventional field effect transistors andfloating gate/node memory cells are reduced in length, the devices beginto be adversely affected by what has been called the short channellength effect. With the short channel length effect, as the channel isreduced in length, the distance between the source and drain regions isdecreased and the impurity diffusions that form the source and drainregions begin to merge under the device in the channel region. This hasthe effect of changing the device into a depletion mode device, where achannel of minority carriers is already formed in the channel regionunder the device and the device conducts current without the applicationof a voltage on the control gate. This leakage current throughunselected devices can corrupt data reads on the common bitlines unlessthe device is isolated or driven with a voltage to force it to turn off.Eventually the short channel effect, with shorter and shorter channels,can progress to the point where the device cannot be shut off,destroying the ability of the control gate to modulate the conductanceof the device (it acts more and more as a resistor) and destroying it asa non-linear device. The short channel effect is typically not seen indual gated transistors and back-side trap memory cell FETs, due to thethin channel body of the device and incident top and bottom fields.

As stated above, both the high voltage requirements and higher leakagecurrents associated with the programming (write/erase) of the currentgeneration of flash technology have begun to adversely affect endurance,reliability, power and speed of operation of the resulting devices andlimit scalability. The high programming and erase voltages apply highfields across the gate insulator stack typically cause gate insulatoroxide degradation. This gate insulator oxide degradation impacts devicenon-volatility (charge retention) and limits the overall deviceendurance (the number of program/erase cycles possible before failure).The high fields also severely limit the amount the device featuregeometry can shrink beyond the current generation due to insulation anddesign requirements. The high programming voltages are also known toinduce a strong capacitive cross-coupling between a selected bit andadjacent unselected bits with the same wordline (or bitline) or betweenadjacent wordlines or bitlines. This cross-coupling has become acritical issue in the overall memory device speed and scaling.Cross-coupling issues typically increase as voltage levels are increasedor as the memory device features are scaled smaller without there beinga commensurate reduction in voltage levels.

As stated above, typical SONOS floating node memory cell devices, unlikefloating gate devices, hold charges in discrete traps associated with anitride trapping layer. The equivalent EOT of the central ONO insulatorstack for such devices can be as low as about half that of floating gatedevice and, therefore, the programming voltage level for a SONOS deviceis about half of that of an equivalent floating gate device. However,further voltage and feature scaling for SONOS devices are limitedwithout it adversely affecting charge retention (due to leakage andback-tunneling) and speed (which is typically tunnel oxide thicknessdependent) and logic window (which is typically nitride trapping layerthickness dependent). Even though SONOS devices may operate by tunnelingfor both write and erase, the peak field across the tunnel oxide canstill be very high (typically 10 MV/cm) causing the associated highfield degradation of the tunnel oxide, adversely affecting endurance andreliability.

Because of the above stated reasons, oxide-based non-volatile memorycell devices, such as traditional Flash, SONOS, or Nano-crystal memorycells are limited in voltage, power, speed, and feature scalability.Additionally, because of the high fields required across the oxideinsulation layers, such oxide-based devices are also limited inreliability and endurance.

Dual gate and Back-side trap devices, because of their structure,sandwich a (typically thin) body region between two gate stacks (the topand bottom gate stacks). This thin body transistor structure preventsshort channel effect by limiting the channel body thickness and theavailable body charge. In addition, dual gate devices have been utilizedin fast logic devices because of their improved channel conductance (dueto twin minority channel carrier regions being formed at both the topand bottom of the channel body) and reduced capacitance therebyenhancing switching characteristics. As a result of these properties,dual gate and back-side trap devices can typically be scaled furtherwhile exhibiting better performance characteristics than single gatedevices.

Back-side trap memory cell embodiments of the present invention utilizeband-gap engineered gate-stacks that allow low voltage program and eraseof the memory cells via the direct tunneling of carriers to or from thetrapping layer held below the channel in the bottom gate stack. Theband-gap engineered bottom gate stack of embodiments of the presentinvention incorporate tunnel insulator layers having asymmetric tunnelbarriers and one or more layers of direct tunnel insulator layers ofincreasing conduction band offset (each following tunnel layer has ahigher conduction band energy level than the previous layer) andincreasing values of K (dielectric constant) to provide very highelectron current density during programming at reduced voltage dropacross the direct tunnel layers. The combination of deep charge trappingcenters and asymmetric tunnel barriers of increasing band offsetprovides large reverse tunneling barrier to promote the required chargeretention. In embodiments of the present invention, appropriateselections of charge trapping material and embedded nanocrystalsfacilitate a desired Vt shift for minimum adequate logical state/logicalwindow separation. In addition, in embodiments of the present invention,the dielectric constant values of the gate stack layers help minimizethe EOT of the gate insulator stack. This enables a gate stack EOT inthe range of 2 nm to 6 nm to allow for low voltage operation and speed.Such methods of direct tunnel programming and erasure utilizing band-gapengineered asymmetric tunnel layers are detailed in U.S. patentapplication Ser. No. 11/131,006, titled “A NOVEL LOW POWER NON-VOLATILEMEMORY AND GATE STACK,” filed May 17, 2005, now issues as U.S. Pat. No.7,612,403, and U.S. patent application Ser. No. 11/127,618, titled“BAND-ENGINEERED MULTI-GATED NON-VOLATILE MEMORY DEVICE WITH ENHANCEDATTRIBUTES”, filed May 12, 2005, now issued as U.S. Pat. No. 7,279,740,both of which are commonly assigned.

As stated above, in direct tunneling of carriers, the carriers arequantum mechanically tunneled into the trapping layer under low energyconditions. To overcome limitations of reverse direct tunnelingoperation, the gate-stack of the embodiments of the present inventionutilizes one or more layers of material that are band-gap engineeredwith increasing band-gap offsets and high K values to form an asymmetricband-gap tunnel insulator layer. This asymmetric band-gap tunnelinsulator layer is very efficient in charge transport in one directionand yet becomes very retarded in the reverse transport direction,presenting a large barrier. The asymmetric band-gap tunnel insulatorlayer allows for a low voltage direct tunneling to the trapping layer ofthe device when a programming field is applied across it utilizing thestepped internal fields of the one or more band-gaps of the asymmetricband-gap tunnel insulator layers, while the same stepped band-gapoffsets and high K dielectrics present a large band-gap and consequentlylarge energy barrier to the charges so trapped to prevent back-tunnelingand maintain the required long term charge retention.

Because of this direct tunneling programming and erasure, embodiments ofthe present invention provide a non-volatile memory cell having aninsulator stack with a low overall EOT, allowing them to be operated atvery low power and at low programming voltage. The low voltage programand/or erase operations of embodiments of the present invention alsoreduce damage in the material of the device due to thetunneling/injection of carriers through the insulator layers (theinjected carriers are “cool” and never gain enough energy to affect thelattice or material bonds). In addition, embodiments of the presentinvention allow for improved scaling by allowing the memory to employsmaller effective oxide thicknesses (EOT) and low voltage layout anddesign in the array and support circuitry of the resulting memorydevice. The band-gap engineered tunneling medium of embodiments of thepresent invention consists of one or more layers of direct tunnel layersof increasing conduction band offset and increasing values of K(dielectric constant) to provide very high electron current densityduring programming at reduced voltage drop across the direct tunnellayers, allowing for high speed, low power programming.

For a back-side trap memory cell, the charge blocking layer and tunnellayer placement are positioned in the bottom gate-insulator stack suchthat the tunnel insulator is positioned below the channel body andlocated between it and the floating node/trapping layer, and the chargeblocking layer is between the back-side gate/substrate and the trappinglayer. In this arrangement, charge transport takes place primarilybetween the channel and the trapping layer (floating node) of the bottomgate stack during programming and erasure. In programming operation, thestepped band-gap arrangement facilitates direct tunneling of electronslayer to layer from the channel to the trapping layer with a low appliedfield. After tunneling to the trapping layer, the combined asymmetricstepped energy barriers, long back tunnel distance, and optional deeplevel charge traps act to reduce charge leakage to the substrate andprovide adequate charge retention for non-volatile usage. A high Kcharge blocking layer is also integrated in the bottom gate-stackbetween the trapping layer and the back-side gate/substrate for oneembodiment of the present invention to maintain low charge leakage tothe back-side gate/substrate from the trapping layer.

In another embodiment of the present invention, nano-crystals areembedded in a trapping layer which contains deep high density traps toprovide a large trapped charge density, enhancing logic level separationand increasing stored charge, while minimizing the adverse effects ofcoulomb blockade and quantum confinement. The use of deep traps and/ornano-crystals further increase charge retention by providing deepquantum wells at the trapping layer, further increasing the potentialbarrier that the trapped charges must overcome to escape from thetrapping layer or back-tunnel through.

During read operations, the front/access gate is active and asserts afield to generate a channel of minority carriers in the channel bodyregion, operating the top gate stack as a conventional FET. The bottomgate stack is inactive during reads, aside from the field asserted bythe charge trapped on the trapping layer on carriers in the channel.

Programming the back-side trap memory cell embodiments of the presentinvention is accomplished by providing a voltage across the channel tothe back-side gate/substrate to apply a field across the one or moretunnel layers and induce direct tunneling of electrons from the channelto the trapping layer of the bottom gate stack.

In multi-bit memory cell programming, multiple data bits are encodedinto a memory cell usually via either multiple charge centroids storedin the trapping layer or by modulating the threshold voltage level toencode the stored data bits. In threshold voltage modulated multi-bitstorage, also known as multi-level cell (MLC) storage, differingthreshold voltage levels are utilized to encode the data values storedin the memory cell, thus making a large logic window separation in thememory cell advantageous. The memory cell is then read by sensing thethreshold voltage the memory cell activates at. In charge centroidmulti-bit storage, the voltage is applied between a selectedsource/drain region (with back-side trap memory cell operating with theselected source/drain region acting as a source and the secondsource/drain region acting as the drain) and the back-sidegate/substrate, tunneling electrons to the trapping layer directlyadjacent to the selected source/drain region. The memory cell is thenread utilizing the access gate and top gate stack by reversing theoperational function of the first and second source/drain regions (theselected source/drain region acting as the drain and the secondsource/drain region acting as the source).

Erasure in back-side trap memory cell embodiments of the presentinvention is also accomplished by direct tunneling of holes and byenhanced Fowler-Nordheim tunneling of electrons from the trapping sites.A voltage is applied across the tunnel layers from the back-sidegate/substrate to the channel, applying a field across the one or moretunnel layers and inducing direct tunneling of holes from the channelbody and Fowler-Nordheim tunneling of electrons from the trapping layerof the bottom gate-stack to erase the memory cell. The erasure operationis relatively slower than the corresponding direct tunnel programoperation, due to the asymmetric tunnel barrier and/or the highereffective mass of holes. The relatively slower erase, however, can beoffset by utilization of a block erase operation, where a large block ofbits is erased in parallel.

It is noted that the erase speed of the back-side trap memory cellerasure of embodiments of the present invention can also be accomplishedby or enhanced by a combination of conventional hot-hole injection,enhanced Fowler-Nordheim electron tunneling, or by enhancedFowler-Nordheim tunneling of holes from the back-side gate/substrate.

It is also noted that the appropriate selection of back-sidegate/substrate passivation layers as well as band engineered tunnellayers can be utilized to enhance erase speed through simultaneous holeinjection from the back-side gate/substrate during erase operation. Suchmethod of memory cell erasure through hole injection is detailed in U.S.patent application Ser. No. 11/127,618, titled “BAND-ENGINEEREDMULTI-GATED NON-VOLATILE MEMORY DEVICE WITH ENHANCED ATTRIBUTES”, filedMay 12, 2005, now issued as U.S. Pat. No. 7,279,740, and U.S. Pat. No.6,784,480, titled “ASYMMETRIC BAND-GAP ENGINEERED NONVOLATILE MEMORYDEVICE,” issued Aug. 31, 2004, both of which are commonly assigned.

This programming and erasure by the transport of electrons and holes bydirect tunneling allows embodiments of the present invention to consumeorders of magnitude lower power compared to conventional flash memorycells and devices. Writing and erase speed are significantly enhanced aselectrons and holes tunnel directly from one direct tunnel layer intothe next through successive layers of low barrier energy.

As stated above, the tunnel insulation layer region of embodiments ofthe present invention may consist of one or more layers of dielectricmaterial with increasing conduction band offsets and/or increasingdielectric K values, allowing directionally asymmetric direct tunnelingefficiency of carriers through the tunnel layer. The layers ofdielectric material can be selected from any commonly utilized insulatormaterials (oxides, mixed oxides, nitrides, oxynitrides, mixedoxynitrides, or silicates) so long as they are arranged in increasingband-gap offsets and, preferably, higher K dielectric materials to helpreduce the EOT of the resulting memory cell. Examples of these insulatormaterials include, but are not limited to, silicon dioxide (SiO₂),titanium dioxide (TiO₂), hafnium oxide (HfO₂), zirconium oxide,Praseodymium Oxide (Pr₂O₃), alumina (Al₂O₃), mixed oxide hafnium andaluminum, mixed oxide of hafnium and titanium, mixed oxide of hafniumand silicon, silicon oxynitride (SiON), SiN, AlN, HfN, mixed oxynitridesof hafnium and silicon, etc. These layers of increasing band-gap offsetdielectric material are typically deposited during manufacturingprocessing utilizing atomic layer deposition (ALD) or other appropriatedeposition processes.

For example, the tunnel layer region may be a composite of one or moremono layers of SiO₂ (band gap 9 eV, K=3.9), formed under the channelregion, followed by one or more mono-layers of silicon nitride (SiN,band offset 1.03 eV, K=7) or alumina (Al₂O₃, band offset: 4.5 eV, K=10)to be followed by a one or more mono-layers of HfO₂ (band offset: 1.65eV, K=24) or Pr₂O₃ (band offset: 1.9 eV; band gap 3.9 eV; K=30) or TiO₂(band offset 3.15 eV; K=60) for a three layer tunnel layer structure. Atwo-layer tunnel structure can consist of SiO₂/Pr₂O₃ or SiO₂/TiO₂,SiO₂/HfO₂, etc. It is noted that other two, three, or more layerasymmetric band-gap tunnel regions of embodiments of the presentinvention are also possible and will be apparent to those skilled in theart with the benefit of the present disclosure, and as such the aboveexamples should not be considered as limiting.

It is also noted that, in one embodiment of the present invention, thesuccessive layers of the asymmetric band-gap tunnel layer not only haveincreasing band offsets, but also have material of higher K values andincreased direct tunnel thickness to minimize the effective EOT of thetunnel layer composite and optimize voltage drop across the each of thetunnel layers. In embodiments of the present invention, the physicalthickness of the composite tunnel layer can be preferably designed to bearound 3 nm or less and the EOT around 1.5 nm or less for low voltageoperation. For example, a typical tunnel layer may consist of 0.8 nm ofSiO₂+1 nm of SiN+1 nm of HfO₂ (EOT=1.6 nm) or 0.8 nm of SiO₂+1 nm ofHfO₂+1.5 nm of Pr₂O₃ (EOT=1.3 nm) or 0.8 nm of SiO₂+1 nm of HfO₂+2 nm ofTiO₂ (EOT=˜1.2 nm).

In order to improve retention and charge density, metal nano-crystalmemory devices have been utilized that contain certain metal orsemiconductor nano-dots or nano-crystals including, but not limited to,tungsten, silicon, germanium, cobalt, platinum, gold, iridium, andpalladium to provide deep energy trapping sites at the metal-insulatorinterface due to large work function difference. However, such devicesrequire adequate dot size and dot separation to ensure effectiveretention of trapped charges due to quantum confinement effect (toprevent electrons from tunneling between adjacent trapping sites withinthe trapping layer or tunneling back to silicon). In addition, coulombblockade (where like charges repel each other) could further degradecharge retention, such that, in design, multiple charge trapping pernano-dot should be avoided.

If coulomb blockade is minimized such that effectively a single electronis captured for every available nano-dot trapping site and the nano-dotsize and separation are optimized to reduce the adverse effects ofquantum confinement, the effective charge trapping density of nano-dottrapping layer is limited to around 1E12/cm² to 2E12/cm² regardless ofthe actual density of nano-dots. Consequently, the effective chargetrapping density for conventional nano-dot or nano-crystal devices islimited. This effective charge trapping density limitation can beovercome if a selected geometry and distribution of nano-crystals wereembedded in insulating trapping layers that also contained a highdensity of naturally occurring deep traps, such as SiN, AlN, or SiON. Ifthis trapping layer is also formed of high K material, the EOT of theentire bottom gate stack would also be reduced.

The above concept is utilized for the trapping medium in one embodimentof the invention. In this approach, the trapping medium may consist ofan appropriate thickness of an insulator having a large number ofnaturally occurring trap sites, such as silicon oxynitride (SiON, trapdepth: Et>1.2 eV, refractive index ˜1.8, K=7) or HfO₂ (trap depth:Et=1.5 eV, K=24), silicon nitride (Si₃N₄, trap depth: Et=1.0 eV,refractive index=2.0, K=7), silicon-rich silicon nitride, tantalumoxide, (Ta₂O₅, Et=2.7 eV, K=26) aluminum nitride (AlN, trap depth>1.0eV, K=10) or TiO₂ (trap depth: Et=0.9 eV; K=60). The trapping medium isthen embedded with nano-crystals/nano-dots that can include, but are notlimited to tungsten, silicon, germanium, cobalt, platinum, iridium,gold, or palladium in sizes ranging from 1.5 nm to 4 nm with aseparation of 3.5 nm to 5 nm to further increase the number of trappingsites.

As stated above, the silicon oxy-nitride (SiON) utilized in the aboveexample provides additional charge trapping sites. The nitrogen-richSiON has an atomic silicon concentration of approximately 38%-40%, anatomic oxygen concentration of approximately 20%, and an atomic nitrogenconcentration of approximately 40%, resulting in a trapping layer with adielectric constant of approximately 7, refractive index ofapproximately γ=1.8, a band-gap of approximately 5.5 eV to 5.7 eV, and acharge trap density of 8E12-1E13/cm2, with a trap depth of approximately1.7 eV. In the above SiON, deep energy traps are associated with largerconcentrations of Si—O—N bond “defects” in the silicon oxy-nitrides.Such a trapping layer of an embodiment of the present invention wouldprovide an effective charge density in the desired range of 5E12/cm² to1E13/cm² without the adverse effects on retention due to coulombblockade or quantum confinement. It is noted that other charge trappinginsulator materials may also be utilized in embodiments of the presentinvention as the charge trapping layer. Such a nitrogen-rich SiONtrapping layer can also be combined with a oxygen-rich siliconoxy-nitride, SiON, (refractive index of approximately γ=1.55, band-gap7.3 eV, and K=5) tunnel layer to provide an asymmetric tunnel barrier ina single layer. The oxygen-rich silicon oxy-nitride (SiON, withrefractive index of approximately γ=1.55) has an atomic oxygenconcentration of >=46%, while its atomic silicon concentration is <=33%.

For one embodiment of the present invention, the charge blocking layersare preferably comprised of a single or composite layer of large Kdielectric material layers such as Al₂O₃ (K=10) or Pr₂O₃ (K=30) or TiO₂(K=60) of thickness greater than 6 nm, to provide a large electronenergy barrier and prevent direct tunneling of trapped charges to theback-side gate/substrate while helping minimize the overall EOT of thebottom gate-stacks. As stated above, charge blocking layers have beendisclosed that allow for erasure of the data stored in the trappedcharge on the trapping layer of the memory cell by allowing thetunneling or injection of hole or electron carriers through the chargeblocking layer from the back-side gate/substrate. It is noted, however,that multiple insulators can be utilized in charge blocking layers ofembodiments of the present invention, including, but not limited toinsulators from the oxide, mixed oxide, nitride, oxynitrides, mixedoxynitrides, and silicate families.

The access gate and back-side gate/substrate of embodiments of thepresent invention typically have a thin passivating conductiveunderlayer of HfN, TiN, IrO₂ or TaN (for process integration) over thegate or substrate. The access gate or back-side gate (if a separatematerial and not formed from the substrate) typically comprise either apolysilicon gate or any other appropriate metal gate (such as aluminum,iridium or tungsten) formed under the charge blocking layer of thebottom gate stack.

The total EOT for a gate stack of an embodiment of the present inventionmade with the above materials and specifications will typically rangefrom EOT=2.5 nm to EOT=6.5 nm with physical thickness ranging (excludingthe back-side gate electrode thickness) from 10 nm and upward, have aprogramming voltage as low as 1.5V, and an average field as low as1.0E6V/cm². This allows back-side trap memory cells and devices ofembodiments of the present invention to provide voltage scalability andlow power consumption levels not available in other current memorydevices.

FIGS. 1A and 1B detail physical cross sections of both a bulk siliconimplementation 100 and a silicon-on-insulator (SOI) implementation 140of the back-side trap non-volatile device structures, while FIG. 1Edetails the corresponding band-gap diagram of a back-side trap memorycell 100, 140 in accordance with embodiments of the present invention.In FIG. 1A, a bulk back-side gate NFET memory cell 100 is shown formedon a substrate 102. The memory cell 100 has a first and secondsource/drain regions 104, 106, in contact with a thin or thick bodychannel region 108. A top gate-stack 110 is formed over the silicon bodychannel region 108, containing an access gate 114 isolated from the bodychannel 108 by a gate insulator layer 116. A bottom gate-stack 112 isformed under the channel region 108. In the bottom gate-stack 112, theback-side control gate 122 is formed as a bulk back-side control gatefrom the substrate 102, which may either be a bulk orsilicon-on-insulator (SOI) substrate. The insulator stack 120 of thebottom gate-stack 112 contains a charge blocking layer 124 formed overthe bulk back-side control gate 122, a trapping layer/floating gate 126formed over the charge blocking layer 124, and a tunnel insulator layer118 formed over the trapping layer 126, directly under the channelregion 108. The tunnel insulator layer 118 includes one or more layersof materials layered to provide an asymmetric stepped band-gap profile.The trapping layer 126 may also be optionally provided with deep trapsand embedded metal nano-crystals.

Multiple layers of single crystal semiconductor films can be depositedover bulk silicon substrate, as well as over SOI substrates. Forexample, a layer of appropriate thickness germanium can be depositedover a silicon substrate to be followed by another layer of epitaxiallygrown silicon. By a combination of lithographic patterning and selectiveetching of underlying germanium films, selective localized voids can beformed underneath the top silicon film. This technique was developed byM Jurezak et al. VLSI Tech. Digest p. 29, 1999, and is calledsilicon-on-nothing or SON. Recently R. Ranica et al. built andcharacterized PMOS back-side trapped SONOS memories by depositing ONOlayers on SON (R. Ranica et al., IEEE Silicon nanoelectronic workshop,p. 99, 2004). The present invention applies similar techniques in bulksilicon and SOI to create novel back-side trapped non-volatile memorycells.

For another embodiment of the present invention, a back-sidenon-volatile memory cell comprises a first and second source/drainregions formed over the buried oxide (BOX) region of an SOI substrate.Top gate and underlying trap-free insulator is formed over the thin-bodyp-type silicon similar to a fully depleted SOI-NFET device. Underneaththe thin floating body, an ONO replacement back gate-insulator stack andthe back gate is formed between the thin body and the BOX region. Theback gate may consist of heavily doped N+ or P+ polysilicon or metal,while the ONO replacement layer consists of a charge blocking layerformed over the back gate, a trapping layer formed over the chargeblocking layer, and one or more sub-layers of tunnel insulator formedover the trapping layer and interfacing the thin floating body. The backgate and the insulator stack are formed in a similar manner ofsilicon-on-nothing (SON) implementation as outlined by the reference ofR. Ranica et al. The tunnel insulator layer may consist of one or morelayers of dielectric material of increasing conduction band offset. Thethin body implementation, as described above, provides immunity fromshort channel effects and aids in further feature size reduction.

In FIG. 1B, a silicon-on-insulator (SOI) back-side gate NFET memory cell140 is shown formed on a substrate 142 utilizing a silicon-on-nothing(SON) design. Silicon-on-nothing (SON) design forms a void 164 under thebody of the device to reduce the effect of substrate parasitics on theresulting circuit element. The memory cell 140 has a first and secondsource/drain regions 144, 146, in contact with a floating body channelregion 148. A top gate-stack 150 is formed over the channel region 148,containing an access gate 154 isolated from the channel by an insulatorlayer 156. A bottom gate-insulator stack 152 is formed under thefloating body region 148. The bottom gate-insulator stack 152 contains acharge blocking layer 162 formed over a back-side gate 166, a trappinglayer 160 formed over the charge blocking layer 162. A multi-layerasymmetric band-gap tunnel insulator layer 158 is formed over thetrapping layer 160, directly under the channel region 148. The tunnelinsulator layer 158 includes two or more layers of materials layered toprovide an asymmetric stepped band-gap profile. The trapping layer 160may also be optionally provided with deep traps and embedded metalnano-crystals.

FIG. 1C illustrates an example of an ONO replacement gate-insulatorstack for the bottom gate-insulator stack 120, 152 of an embodiment ofthe present invention. A single oxygen-rich silicon oxynitride 118, 158(>46 atomic percent oxygen, 20 atomic percent nitrogen, refractive indexγ=1.55, K=5) replaces the tunnel oxide with a barrier height of 2.8 eV,compared to 3.2 eV for SiO₂, thereby enhancing direct tunnel electronfluence during programming. The trapping dielectric 126, 160 is a layerof nitrogen-rich silicon oxynitride (approximately 40 atomic percentnitrogen, 20 atomic percent oxygen, refractive index of approximatelyγ=1.8, K=7) which provides higher density (approximately 1E13/cm²)deeper traps (trap depth of approximately 1.7 eV compared to a nitridetrap depth of approximately 1 eV). The charge blocking layer is replacedby alumina (Al₂O₃) 124, 162 which has barriers comparable to SiO₂ toelectrons and holes and at the same time a K value of approximately 10compared to SiO₂ (K˜=3.9), thereby reducing the overall EOT of theresulting gate-insulator stack. The tunnel oxygen-rich oxynitride(γ=1.55) has the characteristic of very low trap density similar to SiO₂and yet has a significantly lower leakage compared to SiO₂ therebyproviding improved retention compared to an equivalent ONOgate-insulator stack, the above stack would reduce the EOT by two-thirds(⅔×) and consequently reduce write/erase voltage levels while improvingspeed, retention and endurance.

FIG. 1D illustrates the details of a relatively more complex example ofa bottom gate-insulator stack 120, 152 of an embodiment of the presentinvention containing tunnel dielectric medium 118, 158, a more complextrapping medium 126, 160, and a high-K charge blocking medium 124, 162.The tunnel media 118, 158 consists of two or more layers of dielectricof increasing conduction band offset and high K values, enabling directtunnel “barrier thinning” (a shorter effective tunnel distance whenfield is imposed across the tunnel insulator layer) thereby promotingsignificantly enhanced electron transport across the layers at reducedfields. The trapping media 126, 160 consists of a high K dielectriclayer of intrinsic deep traps of high trap density, aided by enhancedtrapping due to embedded high work function nano-dots. The high K chargeblocking layer 124, 162 is alumina (Al₂O₃) 124, 162 which has barrierscomparable to SiO₂ to electrons and holes and at the same time a K valueof approximately 10 compared to SiO₂ (K˜=3.9), thereby reducing theoverall EOT of the resulting gate-insulator stack. The EOT of anequivalent gate-insulator stack can be reduced by 30% or more comparedto an ONO gate-stack and correspondingly lowers the programming voltagelevels.

Many options of ONO replacement may be considered for the bottomgate-insulator stack 120, 152, to achieve (a) a lower EOT of theinsulator stack, thereby achieving voltage scaling; (b) band engineeredasymmetric direct tunneling for electrons and holes to achieve lowvoltage, low energy high speed carrier transport to the trapping sitesand reduce back tunneling of carriers; (c) high density deep energytrapping material to achieve retention and logic window; and (d) high K,large band-gap charge blocking insulator with high barrier energy forelectrons and holes, to prevent back injection and charge loss. For (a),(c) and (d) high K insulators are desired to reduce the incident field,thereby improving stack reliability. For high speed carrier transport inthe desired direction (item (b)), enhancement of direct tunnel orFowler-Nordheim tunneling is required. Direct tunneling is enhanced byreducing tunneling distance (“barrier thinning”) and by reducing barrierenergy while the later enhances Fowler-Nordheim tunneling. Specificexamples of which are detailed herein, in the various embodiments of thepresent invention.

Specifically, in one embodiment, the tunnel insulator layer 118, 158contains three layers of material, a first layer of 0.5 nm of SiO₂(K=3.9), having a band-gap of approximately 9 eV formed under thechannel region 108, 148, which has a band-gap of 1.1 eV. A second layerof 1 nm of SiN (band offset 1.03 eV, K=7) or Al₂O₃, (band gap: 8.8 eV,K=10) is formed under the first layer of SiO₂. And a third layer of 1 nmof HfO₂ (band-gap: 4.5 eV, K=24) is formed under the second layer.

The trapping layer 126, 160 is formed of a layer of 5-7 nm of TiO₂(band-gap of approximately 3.15 eV, K=60) with 3.5-4.0 nm of embeddedCobalt nano-dots, having a resulting EOT of nearly 0.3 nm.Alternatively, the trapping layer could be a single layer of aluminumnitride (AlN, K=15) or nitrogen-rich SiON (of refractive index ofapproximately γ=1.8 and K=7) of appropriate thickness containing deeptraps (Et>1.0 eV). The charge blocking layer 124, 162 is formed of 10 nmof Al₂O₃, (band gap: 8.8 eV, K=10), Pr₂O₃ (band gap: 3.9 eV, K=30), TiO₂(band gap: 3.15 eV, K=60) with an EOT as low as 0.67 nm. The back-sidegate 166 (in embodiments with separate back-side gates) is thentypically formed of polysilicon, tungsten, iridium, or aluminum and mayinclude an initial passivation layer, such as a thin layer of HfN, TaN,TiN, or IrO₂ formed under the charge blocking layer 124, 162.

In another embodiment, the tunnel insulator layer 118, 158 also containsthree layers of material, a first layer of 0.5 nm of SiO₂ (K=4), havinga band-gap of approximately 9 eV formed under the channel region 108,which has a band-gap of 1.1 eV. A second layer of 1 nm of SiN (bandoffset 1.03 eV, K=7) or oxygen-rich silicon oxy-nitride, SiON,(refractive index of approximately γ=1.55, band-gap 7.3 eV, and K=5) orAl₂O₃, (band gap: 8.8 eV, K=10) is formed under the first layer of SiO₂.And a third layer of 1.5 nm of HfO₂ (band-gap: 4.5 eV, K=24) is formedunder the second layer. The effective oxide thickness (EOT) of thesethree layers could be as low as 1.32 nm. The oxygen-rich siliconoxy-nitride (SiON, with refractive index of approximately γ=1.55) has anatomic oxygen concentration of >=46%, while its atomic siliconconcentration is <=33%. The corresponding nitrogen-rich siliconoxy-nitride (SiON, with a refractive index of approximately γ=1.8) hasan atomic oxygen concentration of <=25%, while the atomic nitrogenconcentration is approximately 40%.

The trapping layer 126, 160 is formed of a layer of 6 nm of HfO₂ with3.5-4.0 nm of embedded Cobalt nano-dots, having a resulting EOT of 0.3nm. The charge blocking layer 124, 162 is formed of 10 nm of TiO₂ (K=60)with an EOT of 0.67 nm. And the back-side gate electrode 166 formed of10 nm of TiN as a passivation layer and doped polysilicon.

It is noted, as detailed above, that multiple insulators can be utilizedin embodiments of the present invention, including, but not limited toinsulators from the oxide, mixed oxide, nitride, oxynitride, mixedoxynitride, and silicate families.

FIG. 1E details a band-gap diagram 170 of a back-side trap memory cell100, 140 of FIGS. 1A and 1B in accordance with embodiment of the presentinvention. In FIG. 1E, the access gate 114, 154 is separated from thebody/channel 108, 148 by the access gate insulator layer 116, 156. Thetrapping layer 126, 160 is separated from the channel 108, 148 by theone or more layers of the asymmetric band-gap tunnel layer 118, 158, andfrom the back-side gate 166/substrate 102 by the charge blocking layer124, 162.

FIG. 2 shows a simplified diagram of a system 228 incorporating anon-volatile memory device 200 of the present invention coupled to ahost 202, which is typically a processing device or memory controller.In one embodiment of the present invention, the non-volatile memory 200is a NOR architecture Flash memory device or a NAND architecture Flashmemory device. The non-volatile memory device 200 has an interface 230that contains an address interface 204, control interface 206, and datainterface 208 that are each coupled to the processing device 202 toallow memory read and write accesses. It is noted that other memoryinterfaces 230 that can be utilized with embodiments of the presentinvention exist, such as a combined address/data bus, and will beapparent to those skilled in the art with the benefit of the presentdisclosure. In one embodiment of the present invention, the interface230 is a synchronous memory interface, such as a SDRAM or DDR-SDRAMinterface. Internal to the non-volatile memory device, an internalmemory controller 210 directs the internal operation; managing thenon-volatile memory array 212 and updating RAM control registers andnon-volatile erase block management registers 214. The RAM controlregisters and tables 214 are utilized by the internal memory controller210 during operation of the non-volatile memory device 200. Thenon-volatile memory array 212 contains a sequence of memory banks orsegments 216. Each bank 216 is organized logically into a series oferase blocks (not shown). Memory access addresses are received on theaddress interface 204 of the non-volatile memory device 200 and dividedinto a row and column address portions. In one embodiment of the presentinvention, the non-volatile memory 200 is utilized by the host 202 as auniversal or ideal memory, replacing both the RAM and ROM in the system228.

On a read access the row address is latched and decoded by row decodecircuit 220, which selects and activates a row/page (not shown) ofmemory cells across a selected memory bank. The bit values encoded inthe output of the selected row of memory cells are coupled to a localbit line (not shown) and a global bit line (not shown) and are detectedby sense amplifiers 222 associated with the memory bank. The columnaddress of the access is latched and decoded by the column decodecircuit 224. The output of the column decode circuit 224 selects thedesired column data from the internal data bus (not shown) that iscoupled to the outputs of the individual read sense amplifiers 222 andcouples them to an I/O buffer 226 for transfer from the memory device200 through the data interface 208.

On a write access the row decode circuit 220 selects the row page andcolumn decode circuit 224 selects write sense amplifiers 222. Datavalues to be written are coupled from the I/O buffer 226 via theinternal data bus to the write sense amplifiers 222 selected by thecolumn decode circuit 224 and written to the selected non-volatilememory cells (not shown) of the memory array 212. The written cells arethen reselected by the row and column decode circuits 220, 224 and senseamplifiers 222 so that they can be read to verify that the correctvalues have been programmed into the selected memory cells.

As previously stated, the two common types of EEPROM and Flash memoryarray architectures are the “NAND” and “NOR” architectures, so calledfor the similarity each basic memory cell configuration has to thecorresponding logic gate design. In the NOR array architecture, thenon-volatile memory cells of the memory array are arranged in a matrixsimilar to RAM or ROM. The access gates of each non-volatile memory cellof the array matrix are coupled by rows to word select lines (wordlines) and their drains are coupled to column bit lines. The source ofeach non-volatile memory cell is typically coupled to a common sourceline. The NOR architecture non-volatile memory array is accessed by arow decoder activating a row of non-volatile memory cells by selectingthe word line coupled to their access gates. The row of selected memorycells then place their stored data values on the column bit lines byflowing a differing current from the coupled source line to the coupledcolumn bit lines depending on their programmed states. A column page ofbit lines is selected and sensed, and individual data words are selectedfrom the sensed data words from the column page and communicated fromthe memory.

A NAND array architecture also arranges its array of non-volatile memorycells in a matrix such that the access gates of each non-volatile memorycell of the array are coupled by rows to word lines. However each memorycell is not directly coupled to a source line and a column bit line.Instead, the memory cells of the array are arranged together in strings,typically of 8, 16, 32, or more each, where the memory cells in thestring are coupled together in series, source to drain, between a commonsource line and a column bit line. This allows a NAND array architectureto have a higher memory cell density than a comparable NOR array, butwith the cost of a generally slower access rate and programmingcomplexity.

A NAND architecture non-volatile memory array is accessed by a rowdecoder activating a row of non-volatile memory cells by selecting theword select line coupled to their access gates. In addition, the wordlines coupled to the access gates of the unselected memory cells of eachstring are also driven. However, the unselected memory cells of eachstring are typically driven by a higher access gate voltage so as tooperate them as pass transistors and allowing them to pass current in amanner that is unrestricted by their stored data values. Current thenflows from the source line to the column bit line through eachnon-volatile memory cell of the series coupled string, restricted onlyby the memory cells of each string that are selected to be read. Thisplaces the current encoded stored data values of the row of selectedmemory cells on the column bit lines. A column page of bit lines isselected and sensed, and then individual data words are selected fromthe sensed data words from the column page and communicated from thememory device.

FIG. 3A shows a simplified NOR back-side trap floating node/trappinglayer memory array 300 of a EEPROM or Flash memory device of anembodiment of the present invention. In FIG. 3A, a NOR array 300 couplesback-side trap memory cells 302 of embodiments of the present inventionto bit lines 312, source lines 314, word lines 306, and a substrateconnection 322. In forming the NOR array 300, the bit lines 312 andsource lines 314 are typically formed locally from N+ doped regionsdeposited in the substrate and are separated by a channel region. Eachmemory cell FET 302 has a top gate-insulator stack formed over thechannel region and a bottom gate-insulator stack formed under thechannel region and between the N+ doped regions of a bit line 312 and asource line 314, utilizing the N+ doped regions as a drain and sourcerespectively (it is noted that the source line 314 may be replaced witha second bit line 312 connection in multi-bit cell arrays, so that thecurrent flow through the memory cell may be reversed). As describedabove, the bottom gate-insulator stack is made of an asymmetric band-gaptunnel insulator layer having one or more layers formed beneath thechannel region, a floating node/trapping layer formed under the tunnelinsulator, charge blocking insulator layer formed under the trappinglayer, and a back-side gate formed under the charge blocking insulator.It is noted that in another embodiment of the present invention, thesubstrate takes the place and function of the back-side gate, which isthen not individually formed. The top gate-insulator stack is made of aninsulator layer formed on top of the channel region and an access gate306 (typically formed integral to the word line 306, also known as acontrol gate line) formed over the insulator. In FIG. 3A, back-side gatecontrol lines 316 are shown coupled to the back-side gates of each row,allowing the memory cells of the row to be programmed and erased. It isnoted that, instead of individual back-side gate control lines 316, thesubstrate connection 322 can be utilized to apply programming and erasevoltages to the back-side gate, in particular, where the back-side gateis formed by the substrate. To further enable this mode of operation, inone embodiment of the present invention, each row of memory is formed inisolation trenches, allowing the substrate of each isolation trench tobe individually biased. It is also noted that other NOR architecturememory array 300 configurations incorporating embodiments of the presentinvention are possible and will be apparent to those skilled in the artwith the benefit of the present disclosure.

FIG. 3B details a simplified NAND memory string 320 of a NANDarchitecture EEPROM or Flash memory device of an embodiment of thepresent invention. In FIG. 3B, a series of back-side trap memory cells302 of embodiments of the present invention are coupled together sourceto drain to form a NAND string 320 (typically of 8, 16, 32, or morecells). Each memory cell FET 302 has a top gate-insulator stack formedover the channel region and a bottom gate-insulator stack formed underthe channel region. N+ doped regions are formed between each gateinsulator stack to form the source and drain regions of the adjacentfloating node memory cells, which additionally operate as connectors tocouple the cells of the NAND string 320 together. Optional select gates304, that are coupled to gate select lines, are formed at either end ofthe NAND floating node string 320 and selectively couple opposite endsof the NAND floating node string 320 to a bit line 312 and a source line314. In a NAND memory array, the NAND architecture memory string 320 ofFIG. 3B would be coupled to bit lines 312, source lines 314, word lines306, and a substrate connection 322. As described above, the bottomgate-insulator stack is made of an asymmetric band-gap tunnel insulatorlayer having one or more layers formed beneath the channel region, afloating node/trapping layer formed under the tunnel insulator, chargeblocking insulator layer formed under the trapping layer on top of thesubstrate, which takes the place and function of the back-side gate. Tofurther enable this mode of operation, in one embodiment of the presentinvention, each NAND architecture memory string 320 of memory is formedin an isolation trench, allowing the substrate of each isolation trenchto be individually biased for programming and erasure. It is noted thatin another embodiment of the present invention, an individual back-sidegate can be formed under the charge blocking insulator and coupled to aback-side gate control line. The top gate-insulator stack is made of aninsulator layer formed on top of the channel region and an access gate306 (typically formed integral to the word line 306, also known as acontrol gate line) formed over the insulator. In FIG. 3B, the substrateconnection 322 is shown coupled to the back-side gates of each NANDstring 320, allowing the memory cells of each NAND string 320 to beprogrammed and erased. It is noted that, as described above, instead ofutilizing the substrate connection 322, individual back-side gatecontrol lines can be formed and utilized to apply programming and erasevoltages to a formed back-side gate.

It is also noted that other memory cells, memory strings, arrays, andmemory devices in accordance with embodiments of the present inventionare possible and should be apparent to those skilled in the art withbenefit of the present disclosure.

CONCLUSION

Non-volatile memory devices and arrays have been described that utilizeback-side trapped floating node memory cells with band-gap engineeredback-side gate-insulator stacks with asymmetric tunnel barriers.Embodiments of the present invention allow for direct tunnel programmingand efficient erase with electrons and holes, while maintaining highcharge blocking barriers and deep carrier trapping sites for good chargeretention and reduces the possibility of damage to the channel/insulatorinterface by separating sensing from programming via a front side FETelement of the memory device. The direct tunneling program and efficienterase capability reduces damage to the back-side gate stack and enhancesdevice endurance and reliability. Back-side trap memory cells alsoenable scaling of feature size due to voltage scalability andelimination of short channel effect via a thin body channel device.Memory device embodiments of the present invention are presented thatare arranged in NOR or NAND memory architecture arrays. Memory cellembodiments of the present invention also allow multiple levels of bitstorage in a single memory cell, and allow for programming and erasewith reduced voltages.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement, which is calculated to achieve the same purpose,may be substituted for the specific embodiment shown. This applicationis intended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

1. A method of forming a back-side trap non-volatile memory cell,comprising: forming a trapping material wherein the trapping materialhas a conduction band offset; forming two or more sub-layers ofdielectric material on the trapping material, wherein each sub-layer ofdielectric material has a conduction band offset and wherein theconduction band offset of each sub-layer of dielectric material is lessthan the conduction band offset of the material upon which it is formed;and forming a channel region on the two or more sub-layers of dielectricmaterial.
 2. The method of claim 1, further comprising forming a firstand a second source/drain region coupled by the channel region, forminga charge blocking material beneath the trapping material, and forming aback-side gate beneath the charge blocking material.
 3. A method offorming a back-side trap non-volatile memory cell, comprising: forming atrapping material wherein the trapping material has a conduction bandoffset; forming two or more sub-layers of dielectric material on thetrapping material, wherein each sub-layer of dielectric material has aconduction band offset and wherein the conduction band offset of eachsub-layer of dielectric material is less than the conduction band offsetof the material upon which it is formed; and forming a channel region onthe two or more sub-layers of dielectric material; wherein forming twoor more sub-layers of dielectric material further comprises forming afirst sub-layer on the trapping material and forming a second sub-layeron the first sub-layer wherein the first sub-layer conduction bandoffset is greater than the second sub-layer conduction band offset. 4.The method of claim 3, wherein forming two or more sub-layers ofdielectric material further comprises forming a third sub-layer directlyon the second sub-layer wherein the second sub-layer conduction bandoffset e is greater than the third sub-layer band-gap value.
 5. Themethod of claim 1, wherein each of the two or more sub-layers isselected from a group of materials consisting of oxides, mixed oxides,nitrides, oxynitrides, mixed oxynitrides.
 6. A method of forming aback-side trap non-volatile memory cell structure, comprising: forming afirst and second source/drain regions on a substrate, the first andsecond source/drain regions defining an intervening channel region;forming a bottom gate-stack, the bottom gate-stack comprising, forming acharge blocking layer over the substrate, forming a trapping layer overthe charge blocking layer, the trapping layer having a conduction bandoffset, and forming a tunnel insulator layer of two or more sub-layersover the trapping layer and beneath the channel region, wherein eachsub-layer of the tunnel insulator layer has a conduction band offset andwherein the conduction band offset of each sub-layer of the tunnelinsulator layer is less than the conduction band offset of the materialupon which it is formed; and forming a top gate-stack, the topgate-stack comprising, forming an access gate insulator layer over thechannel region, and forming an access gate over the access gateinsulator layer.
 7. The method of claim 6, wherein forming a tunnelinsulator layer and forming a trapping layer further comprises forming atunnel insulator layer of a layer of oxygen-rich SiON (refractive indexof approximately 1.5) and forming a trapping layer of nitrogen-rich SiON(refractive index of approximately 1.8).
 8. The method of claim 6,wherein forming a tunnel insulator layer of two or more sub-layersfurther comprises, forming two or more sub-layers of dielectric materialof increasing conduction band offset, wherein each of the two or moresub-layers of dielectric material are selected from one of an oxide, amixed oxide, a nitride, a oxynitride, a mixed oxynitride, and asilicate.
 9. The method of claim 8, wherein forming a tunnel insulatorlayer of two or more sub-layers further comprises, forming twosub-layers, where a first and second sub-layers are one of SiO₂ andPr₂O₃, SiO₂ and TiO₂, and SiO₂ and HfO₂.
 10. The method of claim 6,wherein forming a tunnel insulator layer of two or more sub-layersfurther comprises, forming three sub-layers, where a first, second, andthird sub-layers are one of SiO₂, SiN, and HfO₂; SiO₂, HfO₂, and Pr₂O₃;and SiO₂, HfO₂, and TiO₂.
 11. The method of claim 6, wherein forming atrapping layer further comprises forming one of a floating gate, afloating node, and an embedded trapping layer.
 12. The method of claim11, wherein forming a trapping layer further comprises forming chargetrapping deep potential wells.
 13. The method of claim 11, whereinforming a trapping layer further comprises forming a trapping layer ofone of oxygen-rich silicon oxy-nitride (SiON), nitrogen-rich siliconoxy-nitride (SiON), aluminum nitride (AlN), silicon nitride (SiN),silicon-rich nitride (SRN), hafnium oxide (HfO₂), tantalum oxide(Ta₂O₅), and titanium oxide (TiO₂).
 14. The method of claim 6, whereinforming a charge blocking layer further comprises forming a chargeblocking layer of one or more sub-layers of high K dielectric.
 15. Themethod of claim 14, wherein forming a charge blocking layer of one ormore sub-layers of high K dielectric further comprises forming the oneor more sub-layers from an insulator from one of oxide, mixed oxide,nitride, oxynitride, mixed oxynitride, and silicate families.
 16. Themethod of claim 15, wherein forming a charge blocking layer of one ormore sub-layers of high K dielectric further comprises forming the oneor more sub-layers of one of hafnium oxide (HfO₂), alumina (Al₂O₃),praseodymium oxide (Pr₂O₃), and titanium oxide (TiO₂).
 17. The method ofclaim 6, further comprising forming a N- or P-doped channel region. 18.The method of claim 6, wherein forming a charge blocking layer over thesubstrate further comprises forming a charge blocking layer over aback-side gate.
 19. A method of operating a back-side trap non-volatilememory cell, comprising: programming the back-side trap non-volatilememory cell via direct tunneling of carriers through a tunnel insulatorlayer, wherein the tunnel insulator layer is an asymmetric band-gaptunnel insulator layer having two or more sub-layers formed beneath achannel region of the back-side trap non-volatile memory cell, whereinthe two or more sub-layers comprise layers of material of increasingconduction band offset; and trapping the carriers in a trapping layerformed under the tunnel insulator layer.
 20. The method of claim 19,wherein trapping the carriers in a trapping layer formed under thetunnel insulator layer further comprises trapping the charge localizedproximate to a source/drain region of the back-side trap non-volatilememory cell for multiple bit storage.
 21. The method of claim 19,further comprising erasing the back-side trap non-volatile memory cellby removing the carriers trapped on the trapping layer via one of directtunneling, Fowler-Nordheim tunneling, channel hot electron (CHE)injection, and hot hole injection from the channel region.
 22. Themethod of claim 19, further comprising erasing the back-side trapnon-volatile memory cell by removing the carriers trapped on thetrapping layer via transporting carriers to or from the trapping layerthrough a charge blocking layer formed under the trapping layer.